The invention relates to variable gain logarithmic attenuators/amplifiers, and more particularly to such devices which are more precisely linear than those of the prior art, and still more particularly to such devices which are more easily implemented using CMOS technology than the devices of the prior art.
The closest prior art is set forth in U.S. Pat. No. 5,077,541 entitled "VARIABLE-GAIN AMPLIFIER CONTROLLED BY AN ANALOG SIGNAL AND HAVING A LARGE DYNAMIC RANGE", issued Dec. 31, 1991 and U.S. Pat. No. 5,432,478 entitled "LINEAR INTERPOLATION CIRCUIT", issued Jul. 11, 1995, both by Gilbert, and both assigned to Analog Devices, Inc.
The Gilbert '541 patent discloses a variable gain amplifier including, as a feedback network, a ladder attenuator circuit 22 having a number of "tap" points which are applied as inputs to the + inputs of ten "g.sub.m stages" 36, the outputs of which are connected to a differential amplifier 28 to produce an output signal of the variable gain amplifier. A "swept" control signal is applied between conductors 32 and 34 of a gain control circuit 30 that includes ten transistors 40, nine base resistors R.sub.S, and eight constant current sources I. The gain control circuit, in effect, includes ten "segments", each of which supplies a controllable bias current to a corresponding g.sub.m stage. The gain range of the variable gain amplifier can be viewed as being subdivided into successive segments, each segment having its own g.sub.m stage connected to a corresponding node of the attenuation ladder. Applying the V.sub.CONTROL voltage to the gain control circuit 30 activates each segment amplifier in turn, by increasing and then decreasing the transconductance of that stage. The increasing and decreasing and transconductance occurs in the adjacent g.sub.m stages in succession in an overlapping fashion to provide a smooth reduction in overall gain. The effective "tap" point of the attenuator is said to be continuously "interpolated" between the nodes of the latter circuit by the gain control circuit 30.
Gilbert's '478 patent refers to the logarithmic attenuator of his earlier '541 patent as producing a plurality of overlapping, exponentially varying currents as the control signal is swept through its entire range. The non-linearities in the current waveforms produce a non-linear gain in the g.sub.m stags as a function of the control signal. Gilbert acknowledges that in many applications, however, a more linear current waveform than obtained in his earlier circuit is required. Gilbert proposed to solve the linearity problems which he attributes to the interpolation circuit 30 in his '541 patent by providing a significantly different interpolation circuit 16 which included five sections or "legs" and four pairs of parallel-connected pairs of shunting diodes connected between such legs. For example, one leg includes constant current source I1 and diode D1 connected in series, the adjacent leg includes current source I2 and diode D2 connected in series, and the shunting diodes include parallel-connected diodes D21 and D12 connected between conductors 36 and 38. Conductor 36 also is connected to an output transistor Q1 which produces a bias current I.sub.A that, in effect, "enables" a corresponding transconductance amplifier 14A and controls the transconductance thereof. See FIG. 1 of the '478 patent, in which transconductance amplifier 14A is similar to the first transconductance amplifier 36 in FIG. 1 of the '541 patent. The control nodes 28 and 30 of the interpolation circuit 16 of the '478 patent are connected to current sources 32 and 34, respectively, which "demand" complementary currents through the diodes D1-D5, and hence the output currents I.sub.A,B . . . E, to be piece-wise linear functions of the control signal supplied between control nodes 28 and 30 as the control signal is "swept" through its full range. FIGS. 5A-E and 6 of the '478 patent illustrate the piece-wise linear behavior of interpolation circuit 16 which constitutes the improvement over the less linear circuitry of Gilbert's '541 patent.
While the logarithmic amplifiers disclosed in the Gilbert '541 and '478 patents operate acceptably for implementation in bipolar integrated circuit structures, they are not as well suited for implementation by means of CMOS integrated circuit structures. One reason for this is that the logarithmic attenuators disclosed in the Gilbert '541 and '478 patents are located in the feedback loop of an operational amplifier, and if the circuits disclosed by Gilbert were to be implemented in CMOS technology, the bandwidth would be reduced by this configuration.
Furthermore, the attenuator circuits disclosed in the Gilbert '541 and '478 patents apparently can not be used as simple attenuators; that is, they must be utilized as a feedback component of an amplifier.
It would be desirable to provide a logarithmic attenuator circuit which can be utilized as a simple attenuator otherwise than as a feedback component of an amplifier. It also would be desirable to provide a logarithmic attenuator circuit which is more readily implemented in an integrated circuit using CMOS technology than the logarithmic amplifiers disclosed in the Gilbert '541 and '478 patents.